Magnetic recording medium having an infra-red light transparent balkcoat layer

ABSTRACT

Flexible magnetic recording media consisting essentially of a web-like nonmagnetic substrate, a magnetic layer applied to one main side of the web-like substrate and a backing layer formed on the opposite main side of the substrate from a polymeric binder, at least one filler, at least one auxiliary pigment and a polyolefin having a spherical particle shape.

This application is a continuation of application Ser. No. 07/888,713,filed on May 27, 1992, now abandoned.

The present invention relates to flexible magnetic recording mediaconsisting essentially of a web-like nonmagnetic substrate, a magneticlayer applied to one main side of the web-like substrate and a backinglayer formed on the opposite main side of the substrate from a polymericbinder and nonmagnetic fillers and auxiliary pigments.

It is known that flexible magnetic recording media can be provided withbacking layers containing nonmagnetizable, nonconductive and/orconductive substances.

U.S. Pat. No. 3 293 066 states that electrostatic charges on magnetictapes, which can form in recorders at high tape speeds, can beeliminated by applying conductive backing layers, and furthermore thebacks of the tapes can be made more hard-wearing by means of backinglayers. Furthermore, GB-A 1 197 661 and U.S. Pat. No. 4,135,031 disclosethat the winding properties of magnetic tapes can be improved byapplying backing layers having a predetermined surface roughness. Suchbacking layers are also known for magnetic cards. EP-A 101 020 disclosesspecial binder mixtures which, particularly with the addition of carbonblack, give backing layers which have excellent adhesive strength, arevery hard-wearing and are stable under conditions of high temperatureand humidity.

Such backing layers are of particular importance in video tapes, inparticular in those for the home video sector. Thus, inter alia, U.S.Pat. No. 4,735,325 proposes a backing layer which consists of carbonblack of different particle sizes and of fillers having a Mohs' hardnessof ≧8 dispersed in a polymeric binder, for improving thescratch-resistance and for reducing the number of errors. In addition toimproving the wear properties and reducing the abrasiveness,the:proposed backing layers also serve to reduce the transparency of thetape material to light, which is necessary particularly when such tapesare used on commercial video recorders. EP-A 105 471 proposes for thispurpose a backing layer based on barium sulfate/α-iron(III) oxide withor without the special addition of carbon black. However, video tapestreated in this manner have the disadvantage that they are unsuitablefor a conventional duplication method, thermomagnetic duplication (TMD).In this TMD method, a magnetic recording medium containing chromiumdioxide as magnetic material is brought into contact with a master tapehaving a high coercive force and provided with the recording, and at thesame time the chromium dioxide magnetic layer is heated above the Curiepoint of the chromium dioxide.

During subsequent cooling below the Curie point, the chromium dioxide ismagnetized according to the information pattern of the master tape. Thechromium dioxide magnetic layer is generally heated with the aid of alaser beam (usually a krypton laser having a wavelength of 1064 nm)through the back of the magnetic tape. However, this means that anybacking layer present must not substantially absorb the energy of thelaser beam.

It is an object of the present invention to provide a magnetic recordingmedium whose backing layer meets the requirements with regard tomechanical properties, such as resistance to wear and abrasiveness,while at the same time has sufficient transparency to light so that acorresponding magnetic recording medium can also be used for the TMDmethod.

We have found that this object is achieved by flexible magneticrecording media, essentially consisting of a web-like nonmagneticsubstrate, a magnetic layer applied to one main side of the web-likesubstrate and a backing layer formed on the opposite main side of thesubstrate from a polymeric binder and nonmagnetic additives when thebacking layer is obtained by dispersing a mixture essentially consistingof an organic polymer and, based on the backing layer, from 2.5 to 25%by volume of a filler, from 0.5 to 3% by volume of an auxiliary pigmentand from 1 to 10% by volume of a polyolefin having a spherical particleshape, a density of from 0.9 to 1.0 and a mean particle size of from 1to 1,000 μm and by applying the resulting dispersion to that surface ofthe substrate which is opposite the magnetic layer, in a layer thicknesssuch that, after solidification of the layer, a layer thickness of from0.1 to 2.0 μm results.

Suitable fillers for the purposes of the present invention areparticulate compounds, such as silica, in particular precipitatedsilica, calcium carbonate, barium sulfate and/or gypsum having a meanagglomerate size of from 0.05 to 4 μm. Suitable auxiliary pigments arelikewise particulate compounds selected from the group consisting ofalumina, α-iron(III) oxide, titanium dioxide, zinc ferrite and/orchromium green, having a mean agglomerate size of from 0.1 to 0.5 μm. Anessential component of the backing layer of the novel magnetic recordingmedium is the polyolefin. These polymers have a spherical particle shapewith a mean particle size of from 1 to 1,000 μm, in particular from 8 to800 μm, advantageously from 30 to 500 μm. Low density polyolefins havingan average molecular weight of from 3,000 to 25,000 and a density offrom 0.9 to 1.0 g/cm³ have proven particularly advantageous.

The dispersion forming the backing layer of the novel magnetic recordingmedium is prepared by conventional process.

Suitable organic polymers for the backing layer are the binders knownfor the production of magnetic recording media. These are copolyamides,polyvinylformals, polyurethane elastomers, mixtures of polyisocyanatesand relatively high molecular weight polyhydroxy compounds, vinylchloride polymers containing more than 60% of vinyl chloride buildingblocks, a copolymerized vinyl chloride with one or more unsaturatedcarboxylic acids of 3 to 5 carbon atoms as comonomers orhydroxyl-containing vinyl chloride copolymers which can be prepared bypartial hydrolysis of vinyl chloride/vinyl ester copolymers or directcopolymerization of vinyl chloride with hydroxyl-containing monomers,such as allyl alcohol or 4-hydroxybutyl or 2-hydroxyethyl(meth)acrylate, said polymers being soluble in conventional solvents.Other suitable binders are mixtures of one or more polyurethaneelastomers with polyvinylformals, phenoxy resins and vinyl chloridecopolymers of the abovementioned composition. Particularly preferredorganic polymers are mixtures of polyurethane elastomers with phenoxyresins and of polyurethane elastomers with vinyl chloride polymers.

Cyclic ethers, such as tetrahydrofuran and dioxane, and ketones, such asmethyl ethyl ketone and cyclohexanone, are preferably used as solventsfor the preparation and processing of the polymers. Of course,polyurethanes can also be dissolved in other strongly polar solvents,such as dimethylformamide, pyrrolidone, dimethyl sulfoxide or ethyleneglycol acetate. It is also possible to mix the stated solvents witharomatics, such as toluene or xylene, and esters, such as ethyl or butylacetate.

For dispersing, the particulate components are mixed together with thedissolved organic polymers and conventional dispersants, such as soybeanlecithin, saturated and unsaturated, straight-chain and branched fattyacids, fatty acid salts, quaternary ammonium compounds and phosphoricacid derivatives, and the mixture is processed in known dispersingapparatuses. It may also be advantageous to add conventional lubricants,such as fatty acids, fatty esters, silicone oils or fluorine-basedadditives to these backing layers.

The dispersion is prepared in ball mills or vertical or horizontalstirred ball mills in a conventional manner. The backing layer ispreferably applied by means of engraved rollers. For evaporating thesolvents and drying or curing the backing layer, the latter is passedthrough a heating tunnel. It is possible to apply both magnetic andbacking layer dispersions in one operation or in succession. The coatedfilms can, if required, be calendered and compacted on conventionalmachines by being passed between heated and polished rollers, ifnecessary under pressure. The thickness of the backing layer is lessthan 2.0 μm, in particular less than 1.5 μm, preferably from 0.3 to 0.7μm.

In an advantageous embodiment of the novel magnetic recording media, thebacking layer is composed of from 15 to 25, in particular from 20 to 25,% by 15 volume of a precipitated silica having an SiO₂ content of from98 to 99.5%, a pH of from 5 to 7 and a density of 1.9 g/cm³, from 0.5 to3, preferably from 1.5 to 2.25, % by volume of a cubic zinc ferritehaving a mean particle size of from 0.1 to 0.5 μm, of a spherical α-Fe₂O₃ or of an Al₂ O₃ and from 1 to 10, preferably from 2 to 5, % by volumeof a spherical low density polyolefin having a mean particle diameter offrom 8 to 800 μm, preferably from 30 to 500 μm, and from 20 to 40,preferably from 25 to 35, % by volume of a linear polyesterurethaneobtained from adipic acid, 1,4-butanediol and4,4-diisocyanatodiphenylmethane, from 15 to 25, preferably from 18 to20, % by volume of a polyphenoxy resin obtained from bisphenol andepichlorohydrin and from 10 to 25, in particular from 15 to 20, % byvolume of a polyisocyanate resin. In addition to the amounts, notexceeding 2% by volume, of a known dispersant and of a lubricant, it maybe advantageous also to add not more than 1.5% by volume of a carbonblack to the dispersion.

Because of the special backing layer, the novel magnetic recording mediahave extremely advantageous running behavior. Because the back of suchmagnetic recording media is mechanically stable and hard-wearing,corresponding video tapes have a greatly reduced number of errorscompared with those of the prior art. If the magnetic layer of the novelmagnetic recording media is based on chromium dioxide, such magnetictapes are particularly suitable for the TMD method. The transparency ofthis backing layer to infrared light furthermore ensures that thechromium dioxide in the magnetic layer can be heated above the Curiepoint substantially without loss, ie. without a laser current increaserequired in comparison with magnetic tapes without a backing layer.

The Examples which follow illustrate the invention and compare it withprior art experiments. In the Examples and Comparative Experiments,parts and percentages are by volume, unless stated otherwise.

EXAMPLE 1

3,325 parts of zirconium dioxide spheres having a diameter of 1.0-1.25mm, 71.5 parts of a precipitated silica having a mean agglomerate sizeof 3 μm, 6.8 parts of a cubic zinc ferrite having a mean particle sizeof 0.12 μm, 9.2 parts of a polyolefin having an average molecular weightof 3,000 and a mean spherical particle diameter of 500 μm, 103 parts ofa 14.75% strength solution of a vinyl chloride copolymer having anaverage molecular weight of 35,000 and a hydroxyl content of 1.8% byweight in a mixture of 45.74 parts of tetrahydrofuran and 39.51 parts ofdioxane, 162 parts of a 10.62% strength solution of a linearpolyesterurethane resin, prepared from adipic acid, 1,4-butanediol and4,4-diisocyanatodiphenylmethane, in a mixture of 47.95 parts oftetrahydrofuran and 41.43 parts of dioxane, 2.8 parts of an isomeric C₁₈-carboxylic acid, and 848 parts of a mixture of 455 parts oftetrahydrofuran and 393 parts of dioxane were introduced into abatchwise stirred ball mill having a volume of 10,000 parts. The stirredball mill was then closed and the contents were dispersed for 6 hours.Thereafter, the mill was opened again and 14.52 parts of 9.26% strengthsolution of dibutyltin laurate in a mixture of 48.68 parts oftetrahydrofuran and 42.06 parts of dioxane, 5.7 parts of an 8.5%strength solution of a fluorine additive in a mixture of 49.10 parts oftetrahydrofuran and 42.4 parts of dioxane, 751 parts of a 10.62%strength solution of the linear polyesterurethane resin described abovein a mixture of 47.95 parts of tetrahydrofuran and 41.43 parts ofdioxane, 414.75 parts of a 14.75% strength solution of the vinylcopolymer described above in a mixture of 45.74 parts of tetrahydrofuranand 39.51 parts of dioxane, 1807.5 parts of a mixture of 969.65 parts oftetrahydrofuran and 837.85 parts of dioxane were introduced and millingwas continued for a further 3 hours.

The dispersion was then removed from the mill. In order to crosslink thelayer after application, 30 parts of a 41.6% strength solution of anisocyanate resin obtained from 1 mol of trimethylolpropane and 3 mol oftoluylene diisocyanate in 58.4 parts of a tetrahydrofuran, per 1,000parts of the dispersion, were stirred in for 15 minutes. Afterfiltration through a paper filter, the dispersion was applied to a 15 μmthick polyethylene terephthalate film by means of an engraved roller andwas dried in the drying tunnel of the coating machine. The resultingbacking layer was 0.5 μm thick.

The backing layer was very uniform and devoid of any stripes. Thepigment volume concentration of the silica in the layer was 22.48%, thatof the zinc ferrite was 2,14% and that of the polyolefin was 2.9%. Forfurther processing, the magnetic layer containing CrO_(z) as magneticpigment was then applied in a conventional manner in a thickness of 2.5μm to the film side opposite the backing layer. After calendering, thefilm web was slit into 12.7 mm wide (1/2 inch) tapes. The tapes werethen tested as follows:

the laser current required to obtain satisfactory copies was measured.This must not be higher than for a tape free of a backing layer sincethe laser current requirement limits the life of the laser lamp. Tomeasure the wear-resistance properties of the backing layer, acontinuous loop of the test tape was passed for 8 minutes, under atension of 60 p, over a cleaning fleece with the back facing the fleece(v=20 cm/s). The number of scratches before and after the test wasdetermined optically. The fewer scratches present or formed during thetest, the more hard-wearing is the backing layer. This test is importantfor the operational reliability of the tapes in the TMD copying machine,in which the tapes are constantly passed over a fleece. Deposits lead toerrors and make continuous changing of the cleaning fleece essential.The results are shown in Table 1.

EXAMPLE 2

The procedure was as described in Example 1, except that spherical α-Fe₂O₃ having a mean particle size of 0.3 μm was used instead of a cubiczinc ferrite, and the pigment volume concentration of the silica was24.86%, that of the α-Fe₂ O₃ was 1.05% and that of the polyolefin was2.85%. The backing layer was likewise applied in a thickness of 0.5 μm.The results are shown in Table 1.

EXAMPLE 3

The procedure was as described in Example 1, except that an α-Al₂ O₃having a mean particle size of 0.1-0.2 μm was used instead of the cubiczinc ferrite. The pigment volume concentration of the silica was 24.81%,that of the αAl₂ O₃ was 1.23% and that of the polyolefin was 2.85%. Theresults are shown in Table 1.

EXAMPLE 4

Instead of the VC copolymers stated in Example 1, 107 parts of a 16.83%strength solution of a polyphenoxy resin obtained from bisphenol A andepichlorohydrin and having 6% by weight of hydroxyl groups in a mixtureof 44.62 parts of tetrahydrofuran and 38.55 parts of dioxane wereintroduced in dispersion phase I and a further 431 parts of the samesolution were introduced in phase II. The other parameters remainedunchanged. The pigment volume concentrations in the layer were 21.05%for the silica, 1.99% for the cubic zinc ferrite and 2.71% for thepolyolefin. The results are shown in the table.

EXAMPLE 5

The procedure was as described in Example 4, except that, instead of thepolyolefin stated in Example 1 having an average molecular weight of3,000, 9.2 parts of a polyolefin having an average molecular weight of6,000, a mean spherical particle diameter of 30 μm and a density of 0.92g/cm³ were used. The pigment volume concentrations in the layer were21.06% for the silica, 2.0% for the cubic zinc ferrite and 2.63% for thepolyolefin. The results are shown in Table 1.

EXAMPLE 6

The procedure was as described in Example 4, except that, instead of thepolyolefin stated in Example 1, 9.2 parts of a polymeric, polyolefinicfiller having an average molecular weight of 6,500, a mean sphericalparticle diameter of 300 μm and a density of 0.96 g/cm³ were used. Thepigment volume concentrations were 21.06% for the silica, 2.0% for thecubic zinc ferrite and 2.63% for the polyolefin. The results are shownin Table 1.

EXAMPLE 7

The procedure was as described in Example 4. Instead of the 9.2 parts ofthe polyolefin used in Example 1, 18.7 parts of the same polyolefin wereemployed. The pigment volume concentrations were 20.49% for the silica,1.95% for the cubic zinc ferrite and 5.29% for the polymeric filler. Theresults are shown in Table 1.

EXAMPLE 8

The procedure was as described in Example 4, except that, instead of thepolyolefin stated in Example 1 having a mean spherical particle size of500 μm, 9.0 parts of a polyolefin having a mean spherical particle sizeof 8 μm and a density of 0.94 g/cm³ were used. The pigment volumeconcentration was 21.06% for the silica, 2.0% for the cubic zinc ferriteand 2.63% for the polymeric, polyolefinic filler. The results are shownin Table 1.

EXAMPLE 9

The procedure was as described in Example 4, except that a highlyconductive carbon black having a specific surface area of 1000 m² /g wasconcomitantly used. Instead of the 71.5 parts of the precipitatedsilica, only 67.4 parts of said! silica and 6.8 parts of the cubic zincferrite and 4.5parts of the highly conductive carbon black wereintroduced.

The pigment volume concentration was 20.25% for the silica, 2.05% forthe cubic zinc ferrite, 1.39% for the carbon black and 2.79% for thepolyolefin. The results are shown in Table 1.

EXAMPLE 10

The procedure was as described in Example 4, except that, instead of the71.5 parts of the precipitated silica, 31.8 parts of a finely divided0.11 μm BaSO₄ having an oil absorption of 24 ml/100 g were used. Allother parameters remained unchanged. The pigment volume concentrationwas 13.84% for the BaSO₄, 2.98% for the cubic zinc ferrite and 4.05% forthe polymeric, polyolefinic filler. The results are shown in Table 1.

COMPARATIVE EXPERIMENT 1

A backing layer dispersion was prepared according to Example 1 of EP-A101 020, except that dispersing was carried out in a batchwise stirredball mill having a capacity of 10,000 parts by volume and containing3,325 parts of zirconium dioxide balls having a diameter of 1.0-1.25mm.126.3 parts of carbon black, 63.15 parts of a silica gel treated withorganic substances, 8 parts of cubic zinc ferrite, 2194.5 parts of amixture of 1223.6 parts of tetrahydrofuran and 970.9 parts of dioxane,10.2 parts of stearic acid, 553.3 parts of a 16.36% solution of apolyurethane resin obtained from 44:56 trimethylolpropane/1,6-hexanediolin a mixture of 44.87 parts of tetrahydrofuran and 38.77 parts ofdioxane, and 838.6 parts of a 16.83% strength solution of a polyphenoxyresin obtained from bisphenol A and epichlorohydrin and having 6% byweight of hydroxyl groups in a mixture of 44.62 parts of tetrahydrofuranand 38.55 parts of dioxane were introduced into said mill.

The batch was then milled for 6 hours, after which 1283 parts of a10.25% strength solution of a saturated polyester resin, prepared from1:2 terephthalic/isophthalic acid and ethylene glycol, in a mixture of48.15 parts of tetrahydrofuran and 41.6 parts of dioxane, 871.6 parts ofa 10.62% strength solution of a linear polyesterurethane resin, preparedfrom adipic acid, 1,4-butanediol and 4,4'-diisocyanatodiphenylmethane,in a mixture of 47.95 parts of tetrahydrofuran and 41.43 parts ofdioxane, 11.6 parts of butyl stearate and 478.5 parts of a 41.6%strength solution of an isocyanate resin obtained from 1 mol oftrimethylolpropane and 3 mol of toluylene diioscyanate in 58.4 parts oftetrahydrofuran were added. Stirring was carried out for a further 15minutes, after which homogenization was complete and the backing layerdispersion was filtered through a paper filter and applied to a 15 μmpolyethylene terephthalate film by means of an engraved roller and driedin a drying tunnel of the coating machine. The thickness of the backinglayer was 0.5 pm. The pigment volume concentrations in the layer were14.3% for carbon black, 5.81% for the silica and 0.74% for the cubiczinc ferrite. For further processing, the magnetic layer was applied ina conventional manner in a thickness of 2.5 μm to the film side oppositethe backing layer. After calendering and slitting of the block into 1/2inch wide tapes, the latter were tested as described under Example 1.The results are shown in Table 1.

COMPARATIVE EXPERIMENT 2

A backing layer dispersion was prepared according to Example 1 of EP-A105 471 from 93 parts of BaSO₄ having a mean particle size of 0.08 μm,20 parts of α-Fe₂ O₃ having a mean particle size of 0.1 μm, 85.98 partsof nitrocellulose, 78.33 parts of polyesterurethane, 32 parts of atrifunctional isocyanate resin, 5.81 parts of n-butyl stearate, 11.62parts of myristic acid, 1057 parts of cyclohexanone and 1157.4 parts oftoluene. The 10 backing layer dispersion was applied to give a 0.5 μmthick layer. The pigment volume concentrations in the backing layer were31.64% for BaSO₄ and 6.8% for α-Fe₂ O₃. Comparative Experiment 2 wasfurther processed and tested in the same way as the other Examples. Theresults are shown in Table 1.

COMPARATIVE EXPERIMENT 3

The procedure was as described in Example 1, except that, instead Of thecubic zinc ferrite and the polyolefin, 81.0 parts of the precipitatedsilica and 8.85 parts of the highly conductive carbon black described inExample 9 were used. The 0.5 μm thick layer contained precipitatedsilica in a pigment volume concentration of 21.56% and carbon black in aconcentration of 2.47%. Further processing and testing were carried outas described. The results are shown in Table 1.

COMPARATIVE EXPERIMENT 4

In this Experiment, no backing layer was applied but only the magneticlayer containing CrO₂ as magnetic material having a coercive force of 51kA/m. Further processing and testing were carried out as for the otherExamples. The results are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                 Examples                      Comparative Experiments                         1  2  3  4  5  6  7  8  9  10 1  2  3   4                        __________________________________________________________________________    Laser current in amp                                                                       32 32 32 32 32 32 32 32 33 32 not                                                                              34 35   32                      in standard TMD copying                    pos-                               method                                     sible                              Scratches                                                                     before test  0  0  0  1  2  2  0   4 1  2  1  0  10    2                      after test   0  1  1  2  3  7  2  11 3  5  3  2  >50 >50                      __________________________________________________________________________

EXAMPLE A

An image was recorded on video tapes obtained according to Example 1,Example 4 and Comparative Experiment 4 (magnetic tape without backinglayer) on a standard TMD copying apparatus. The video properties weremeasured by using the video tape of Comparative Experiment 4 as areference tape, ie. at 0 dB. The signal-to-noise ratio (S/N), the colornoise modulation (CNM) and the HF output were measured. The results areshown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Video values*  Example A/1                                                                              Example A/4                                         ______________________________________                                        S/N [dB]       +2         +1                                                  CNM [dB]       +2.5       +2                                                  HF output [dB] +2         +2                                                  ______________________________________                                    

TABLE 2

We claim:
 1. A flexible magnetic recording medium consisting essentiallyof a non-magnetic substrate in tape form having a first main side and asecond main side opposite to the first main side, a magnetic layerapplied to said first main side of said substrate and a backing layerformed on said second main side from a dispersed mixture consistingessentially of an organic polymer, from 2.5 to 25 % by volume of afiller, from 0.5 to 3 % by volume of an auxiliary pigment, and from1-10% by volume of spherical shaped polyolefin particles having a meanparticle size of from 1-1000 μm and a density of from 0.9 to 1.0 g/cm³,said backing layer being obtained by dispersing said mixture andapplying said mixture to said second main side of said substrate,wherein said backing layer has a layer thickness after solidification ofsaid mixture of from 0.1 to 2.0 μm and a transparency to infra-red lightand energy absorbtion enabling transmission of an infra-red laser beamfor heating of said magnetic layer .
 2. A flexible magnetic recordingmedium as defined in claim 1, wherein the polyolefin has a mean particlesize of from 30 to 500 μm.
 3. A flexible magnetic recording medium asdefined in claim 1, wherein the polyolefin is a low density polyolefinhaving an average molecular weight of from 3,000 to 25,000.
 4. Aflexible magnetic recording medium as defined in claim 1, wherein thefiller is at least one particulate compound selected from the groupconsisting of SiO₂, CaCO₃, BaSO₄ and CaSO₄ having a mean agglomeratesize of from 0.05 to 4 μm.
 5. A flexible magnetic recording medium asdefined in claim 1, Wherein the auxiliary pigment is at least oneparticulate compound selected from the group consisting of Al₂ O₃, α-Fe₂O₃, TiO₂, cubic zinc ferrite and Cr₂ O₃ having a mean agglomerate sizeof from 0.1 to 0.5 μm.
 6. A flexible magnetic recording medium asdefined in claim 1, wherein the mixture used for the backing layer alsocontains not more than 1.5% by volume of carbon black.